1 . Field of the Invention
This invention relates to surgically implanted prosthetic devices. More particularly, this invention concerns a dual-locking mechanism bipolar hip prosthesis that allows for articulation between the human acetabulum and the femoral head of a hip replacement prosthesis.
2. Description of Related Art
Acetabular degeneration associated with single component Austin-Moore or Thompson type endoprostheses is well documented. Although these signal component endoprostheses have achieved general acceptance, long-term follow-up results have revealed many problems, including increased acetabulum wear, protrusion, stem loosening, and dislocation. The major complication of increased acetabular wear is believed to be due to high contact stress and excessive friction between the metal femoral head and the natural acetabulum.
Consequently, developers have sought alternative solutions to reduce the friction between prosthesis and acetabulum. This search has produced a number of multi-component femoral bipolar endoprostheses possessing a low-friction joint between a conventional total hip femoral prothesis and a cup component that articulates with the acetabulum. These prosthetic devices fit loosely into the natural acetabulum. They are designed to connect the femoral head and stem prosthesis to the natural acetabulum and to prevent wear of the acetabulum cartilage frequently caused by one-piece total hip prostheses. Because the motion is greater at the inner low-friction prosthesis joint, acetabular articulation and its associated erosion is reduced.
Constructions of this type are shown by way of example in the following U. S. patents:
English, U.S. Pat. No. 4,004,300
D'Errico, U.S. Pat. Nos. 4,044,403 and 4,172,296
Ramos, U.S. Pat. No. 4,380,090
Pappas, U.S. Pat. Nos. 4,624,674 and 4,619,658
Legrand, U.S. Pat. No. 4,676,799
Fichera et al., U.S. Pat. No. 4,714,477
Jurgutis, U.S. Pat. No. 4,728,335
Oh, U.S. Pat. No. 4,770,661
Each of these constructions consists of a cup member having an outer shell for implantation into the skeletal cavity and an inner cavity for receiving a male prosthesis member, usually having a spherical shape. Bipolar endoprostheses possess two distinct characteristics: at least two centers of movement and a smooth outer shell for acetabular articulation.
One of the considerations in designing a bipolar endoprostheses is the relative positioning of the internal (femoral component head to bearing insert) and external (metal cup to acetabulum) centers of articulation. The original prior art bipolar endoprostheses depicted in FIG. 2a were designed with "negative eccentricity" because the external center of articulation (marked "E") was positioned above the internal center of articulation (marked "I").
The configuration resulted in a persistent varus tilt or vertical position of the external acetabular cup, as depicted in FIG. 2a. Negative eccentricity produces a de-stabilizing torque, which reduces internal articulation and provides undesirable load bearing conditions. If the femoral head 6 became impinged against the inner surface of the outer shell, in vivo component separation and disassembly could occur. U.S. Pat. No. 3,813,699 issued to Gilberty and U.S. Pat. No. 4,172,296 to D'Errico are examples of bipolar endoprostheses designed with negative eccentricity.
More recently, conscientious efforts have been made to create endoprostheses with "positive eccentricity," possibly because of reports of persistent problems with dislocation at a rate comparable to more conventional endoprostheses. As depicted in FIG. 2b, positive eccentricity occur when the external cup center ("E") of articulation is positioned below the internal articulation center ("I"). This configuration keeps the external metal cup from falling into either a marked varus or valgus position and is called "self-centering." U.S. Pat. Nos. 4,619,658 and 4,634, 674 issued to Pappas et al. are examples of bipolar endoprostheses designed with positive eccentricity.
As depicted in FIG. 2b, self-centering cups produce a stabilizing torque which creates a consistent cup position, decreasing the chance of femoral head impingement. The stabilizing torque provides a symmetric load-bearing surface, increases inner joint motion and decreases acetabular wear. Thus, positive eccentricity is a desired characteristic in a bipolar endoprosthesis design.
Intraoperative considerations such as ease of prosthetic component assembly and disassembly and the nature of the installation and removal instrumentation are important to bipolar endoprosthesis design. A continuing objective in the design of bipolar endoprostheses is to allow ease of intraoperative assembly while simultaneously providing for a substantially locked component to ensure against accidental in vivo disassembly. Many of the prostheses in the prior art can be assembled easily by "popping" the femoral head into the acetabulum component. These designs risk in vivo separation of components if insertion and removal forces are relatively small.
However, if the endoprosthesis is designed such that its components are not easily separated, then a relatively large assembly force is normally required. In addition, an undesirable large disassembly force is required should a surgeon need to change the femoral head or should revision of the component to a total hip arthroplasty with a permanently fixed acetabular component be necessary. The increased assembly and disassembly force adds time-consuming steps to the surgery, makes reduction after a post-operative dislocation more difficult, and increases the potential morbidity and mortality associated with subsequent surgical corrections.
In order to decrease the potential for accidental in vivo disassembly, yet allow for relatively easy intraoperative assembly and disassembly, several designs have been developed which use a bearing component with a means for locking the hip prosthesis ball component into the acetabular shell. U.S. Pat. No. 4,380,090 issued to Ramos is an example of such an endoprosthesis. Ramos teaches a four-piece artificial hip socket consisting of an acetabular shell, a bearing insert, an annular bearing, and a locking ring. Ramos uses a split-ring bearing and an annular spring locking ring to engage the femoral head. Ramos has the disadvantage of having multiple inserts to assemble and keep track of during surgery.
Other designs using locking rings for multiple component designs are disclosed by U.S. Pat. No. 4,619,658 issued to Pappas et al. and U.S. Pat. No. 4,770,661 issued to Oh. The retaining ring disclosed by Pappas performs two functions. First, it holds a split inner bearing sub-assembly together during prosthesis assembly. Second, it retains the acetabular cup on and over the split bearing insert, preventing separation during joint rotation. However, this structure can at times be too easily disassembled.
The locking ring taught by Oh has an inner annular surface and external threads which are threaded into the acetabular shell. The locking ring utilizes radially extending tabs which are received in a locking groove of the acetabular shell. However, the threaded coupling makes this structure difficult to use during surgery.
Therefore, a prosthesis with relatively easy assembly and disassembly yet capable of producing a substantial locking of prosthetic components is still desired.
In order to remedy the deficiencies of the prior art, it is an object of the present invention to provide a bipolar endoprosthesis which is easily manufactured, has positive eccentricity, and is self-centering. It is also an object of this invention to provide a bipolar endoprosthesis which is quickly and easily assembled and disassembled intraoperatively, and yet provides a substantial locked assembly. Other objects of the present invention are to substantially reduce accidential in vivo disassembly, reduce inventory requirements, and make external reduction possible.